Arnon Bar-David, who heads the Histadrut, Israel’s most powerful trade union, was detained for questioning by police alongside his wife on Monday morning as part of a major probe into bribery and fraud.

According to Israel’s Channel 12 News, over 350 people were questioned Monday, including several mayors and “very senior” players in the Israeli economy, following a raid on the Histadrut’s Tel Aviv offices.

The affair, dubbed “Hand Shaking Hand” by police, is “one of the most serious cases we have encountered,” Israel Police Commissioner Danny Levi said in remarks to fellow officers cited by the channel.